1. Field of the Invention
This invention relates in general to concrete support foundations constructed in-situ particularly useful for supporting tall, heavy or large towers which may be used to support wind turbines, power lines, street lighting and signals, bridge supports, commercial signs, freeway signs, ski lifts and the like. More specifically, the helical anchor foundation of the present invention is useful in supporting such towers in clays, sands, and other soft materials which can be water-bearing and/or too weak to stand or maintain the excavations formed to receive a concrete foundation. Such soils can be found in the Midwest region and coastal regions of North America.
2. Description of Related Art
My earlier U.S. Pat. No. 5,586,417 for tensionless pier foundation, U.S. Pat. No. 5,826,387 for pier foundation under high unit compression, U.S. Pat. No. 6,672,023 for perimeter weighted foundation, U.S. Pat. No. 7,533,505 for pile anchor foundation, U.S. Pat. No. 7,618,217 for post-tension pile foundation, and U.S. Pat. No. 7,707,797 (the '797 patent) for pile anchor foundation disclose post-tension concrete foundations for tower structures, the disclosures of which are expressly incorporated herein by reference as if fully set forth in their entirety. The prior art cited in these patents may also be relevant to the post-tensioned helical anchor and foundation cap of the present invention.
The foundation disclosed in the '797 patent has a circular post-tensioned concrete cap set on or below ground surface. The foundation supports a tower from the upper surface thereof, which tower is attached to the post-tensioned concrete cap by a series of circumferentially spaced tower anchor bolts. The tower anchor bolts extend upwardly through, and are nutted atop, a circular tower base flange at the bottom of the tower and extend downwardly through, and nutted below, an embedment ring near the bottom of the post-tensioned concrete cap. The tower anchor bolts are also sleeved and shielded so as to prevent the concrete from bonding to the anchor bolts. This structure allows the tower anchor bolts to be elongated and post-stressed between the tower base flange and the embedment ring to alleviate bolt cycling and fatigue and allow the tower anchor bolts to be removed and replaced for bolt remediation, extended fatigue life, and greater bolt strength capacity to allow larger improved structures to be supported by the foundation in the future.
In a conventional helical foundation, the helical anchors are elongated pipes which extend upwardly into and connect to an overlying concrete foundation cap and are both compression and tension resisting foundation extensions which alternate between tension and compression as the foundation moves in a rocking fashion. The helical anchor pipes in such foundations are not connected to tensioning bolts and are not post-tensioned.